An adaptive signal processing system is a system that is capable of altering or adjusting its parameters in such a way that its behavior, through contact with its environment, changes to approximate a desired response. A common application is in the field of telephony where problems in acoustic echo cancellation, line echo cancellation and the like readily lend themselves to solutions based on adaptive signal processing techniques. Other fields of use include mechanical systems, radar, sonar, and biological systems.
A commonly used architecture for implementing adaptive systems is the finite impulse response (FIR) filter. The algorithm commonly used for training this type of filter involves adjustment of the parameters based on the error and the derivative of the output with the parameters. This algorithm up until this point has not been available for the training of IIR filters due to the inability to determine the value of the derivatives. The following is a listing of the techniques used to overcome this deficiency in the training of IIR filters.
For example, U.S. Pat. No. 5,638,439 relates to echo cancellation in a transmission line and teaches a method of updating filter coefficients by taking the absolute values of the coefficients and scaling the resulting vector.
U.S. Pat. No. 5,418,849 is directed to a procedure for adapting a recursive filter using an algorithm based on a variation of Kalman's algorithm. The '849 patent modifies the Kalman algorithm by taking the decimation of the square error rather than the voice signal to achieve improved speed of convergence. U.S. Pat. No. 5,337,366 discloses in FIG. 1 a filter (16) comprising a non-recursive portion (18) and a recursive portion (17). Coefficient control stages (19-21) serve to update the filter coefficients. The '366 patent shows the use of finite impulse response (FIR) filter stages (31, 32) to filter the signals prior to handling by the coefficient control stages.
U.S. Pat. No. 5,335,020 describes a ghost canceling application used in video systems. The patent addresses the inherent instability of adapting IIR filters by providing for a step of determining the onset of such instabilities. The determination is accomplished by summing the weighting coefficients; if the sum exceeds 1 then the filter may be unstable, and appropriate action can be taken.
U.S. Pat. No. 5,226,057 discloses a digital notch filter implemented using an adaptive IIR filter. The filter coefficients are updated in accordance with equations disclosed beginning at column 2, line 62 of the reference.
U.S. Pat. No. 4,751,663 is directed to an IIR filter wherein a polynomial multiplies both the denominator and the numerator of the system transfer function to remove a z.sup.-1 term in the denominator. This permits high speed operation with a pipeline processing technique.